Experimentally derived beam quality correction factors for PPC05 and razor nano ionization chambers in proton scanned...

Ionization chamber based reference dosimetry of proton beam requires the use of beam quality correction factors which are obtained experimentally or by Monte Carlo simulation and are available to users in codes of practice and other publications.

The aim of this work is to determine experimental proton beam quality correction factors, $k_Q$ , for two ionization chambers using a portable graphite calorimeter: the IBA reference-class PPC05 chamber and the IBA non reference-class Razor Nano chamber, using a portable graphite calorimeter.

Measurements were performed in scanned clinical proton beams at two clinical centers. Absorbed dose to graphite was determined using a recently developed portable graphite calorimeter. The conversion factors for the absorbed dose to water determination were obtained by Monte Carlo simulations. Finally, the experimental $k_Q$ factors were obtained by comparing ionization chamber readings with the calorimeter, measured using the same conditions, in terms of absorbed dose to water.

For the IBA PPC05 chamber, the experimental values agreed with recommended values tabulated in TRS-398 (rev. 1) within the combined experimental uncertainty for the three residual ranges of 3.4, 13.3 and 27.7 g·cm^-2. For Razor Nano chamber, $k_Q$ values of 1.009 and 1.011 for residual ranges of 13.3 and 27.7 g·cm^-2 have been determined. No recommended values are provided in TRS_398 (rev. 1). The overall budget yielded a combined standard uncertainty in the experimental $k_Q$ determination of less than $1\%$ ( $k=1$ ).

This study demonstrates the feasibility and high precision of using a portable graphite calorimeter for beam quality correction factor determination in the user beam quality. The short room occupation time required (around 3 hours) makes the system useful as well for beam output dosimetry audits without disturbing clinical practice.